19-inch rack
The 19 inch dimension includes the edges or ears that protrude from each side of the equipment, allowing the module to be fastened to the rack frame with screws or bolts.
The height of the electronic modules is also standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or U (less commonly RU).
The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office.
The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets.
But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 191⁄2 inches between centers.
[8][9] They allow for dense hardware configurations without occupying excessive floor space or requiring shelving.
Typically, a piece of equipment being installed has a front panel height 1⁄32 inch (0.031 in; 0.79 mm) less than the allotted number of Us.
Tapping large numbers of holes that may never be used is expensive; nonetheless, tapped-hole racks are still in use, generally for hardware that rarely changes.
Examples include telephone exchanges, network cabling panels, broadcast studios and some government and military applications.
Square-hole racks allow boltless mounting, such that the rack-mount equipment only needs to insert through and hook down into the lip of the square hole.
A key structural weakness of front-mounted support is the bending stress placed on the mounting brackets of the equipment, and the rack itself.
Servers and deep pieces of equipment are often mounted using rails that are bolted to the front and rear posts (as above, it is common for such rails to have an adjustable depth), allowing the equipment to be supported by four posts, while also enabling it to be easily installed and removed.
The amount of motion and resulting stress depends on the structural characteristics of the building and framework in which the equipment is contained and the severity of the earthquake.
Seismic racks rated according to GR-63, NEBS Requirements: Physical Protection, are available,[13] with Zone 4 representing the most demanding environment.
[14][15] GR-3108, Generic Requirements for Network Equipment in the Outside Plant (OSP), specifies the usable opening of seismic-compliant 19-inch racks.
Computer servers designed for rack-mounting can include a number of extra features to make the server easy to use in the rack: When there is a large number of computers in a single rack, it is impractical for each one to have its own separate keyboard, mouse, and monitor.
A standard 19-inch server rack cabinet is typically 42u in height, 600 millimetres (24 in) wide, and 36 inches (914.40 mm) deep.
Cabinets are generally sized to be no wider than the standard 24-inch-wide (610 mm) floor tiles used in most data centers.
Racks carrying telecom equipment like routers and switches often have extra width to accommodate the many cables on the sides.
A top bar and wide foot connect the posts and allow the rack to be securely attached to the floor and/or roof for seismic safety.
Road cases typically have plywood sides laminated with polyvinyl chloride (PVC), extruded aluminum edges, steel corners, handles, and latches.
Touring musicians, theatrical productions and sound and light companies use road case racks.
[21] In 1965, a durable fiber-reinforced plastic 19-inch rackmount case was patented by ECS Composites and became widely used in military and commercial applications for electronic deployment and operation.
[citation needed] Rackmount cases are also constructed of thermo-stamped composite, carbon fiber, and DuPont's Kevlar for military and commercial uses.
These cases are marketed to musicians and entertainers for equipment not subject to frequent transportation and rough handling.
Low-wattage devices may not employ active cooling, but use only passive thermal radiation and convection to dissipate heat.
For rack-mounted computer servers, devices generally intake air on the front and exhaust on the rear.
This prevents circular airflows where hot exhaust air is recirculated through an adjacent device and causes overheating.
Although open-frame racks are the least expensive, they also expose air-cooled equipment to dust, lint, and other environmental contamination.
However, some rack equipment has been designed to make fan replacement easy, using quick-change fan trays that can be accessed without removing the cabling or the device from the rack, and in some cases without turning off the device so that operation is uninterrupted during replacement.
